Very high power densities made available by lasers have made it possible to observe the nonlinear behavior of optical media, such as crystals. Frequency doubling is an example of such nonlinear behavior. Frequency doubling is a specific example of what is known as the sum-frequency generation process, and occurs when an incident radiation of frequency v, on propagating through some crystalline materials, emerges as radiation consisting of a mixture of two frequencies, the original frequency v and a new frequency 2v.
The inverse of the aforementioned sum-frequency process is the optical parametric process, wherein incident radiation having a frequency v, on propagating through a nonlinear medium is converted into two lower frequency (higher wavelength) waves, which are of essentially variable frequency.
Optical parametric systems can be used to generate secondary (parametrically generated) radiation from a monochromatic coherent (incident) primary radiation by means of parametric interaction with an optically nonlinear medium. The secondary radiation has two components, one of which typically has a shorter wavelength than the other, although it is possible that both of the two components can have the same wavelength. This condition is known as "degeneracy". The wavelengths of both secondary radiation components are longer than that of the primary radiation. The wavelength of a secondary radiation component can be freely selected and can be adjusted by a suitable arrangement, typically by rotation of the optically nonlinear medium within the optical resonator of the parametric system. The wavelength of the other component will be determined based on energy conservation. The optical parametric system and be used in combination with a coherent source of primary optical radiation, for example a laser to provide a source of optical radiation whose wavelength can be selected more or less as desired within a desired frequency range.
In a parametric oscillator, the relationship between the "pump" frequency (f.sub.p) of the incident radiation and the "signal" (f.sub.s) and "idler" (f.sub.i) parametrically generated component frequencies is given as: f.sub.p =f.sub.s +f.sub.i
As the OPO is tuned away from "degeneracy", wherein the signal and idler frequencies are equal, the signal and idler wavelengths change, with the former (signal wavelength) decreasing and the latter (idler wavelength) increasing. Further, as the OPO is tuned well away from degeneracy, the signal and idler wavelengths become well separated, a consequence of which is that their indices of refraction will differ.
Usually, crystals are used as the optically nonlinear medium. Examples of crystals exhibiting the desired nonlinear effects are KDP, LiNbO.sub.3, Ba2Na(NbO.sub.3).sub.5 or LiO.sub.3. Since these nonlinear media exhibit relatively weak parametric interaction, it is known to locate the nonlinear medium within an optical resonator formed by mirrors, so that the radiation passes repeatedly through the medium. However, this creates a substantial problem with respect to coupling radiation into and out of the resonator. Hence, it has been known to employ mirrors, defining the resonator, which are highly transmissive for the primary radiation and highly reflective for the secondary radiation. This requirement is difficult to meet, when the parametric arrangement is intended to be continuously tunable regarding the wavelength of the secondary radiation, since the mirrors have to be highly reflective at a region close to the wavelength of the primary radiation, and the range of high reflectivity should be wide over a wide range of wavelengths. In practice, the reflectivity should be on the order of 80 to 95%. Even the most highly developed dichroic mirrors, made of multiple dielectric layers, only partially meet this requirement, and then only at great cost.
U.S. Pat. No. 4,639,923 describes an Optical Parametric Oscillator (OPO) using a urea crystal. This crystal makes the OPO broadly tunable, so that the entire spectral range from the ultraviolet to the near infrared is accessible. Further, the urea OPO is angle-tunable, and has a high efficiency.
U.S. Pat. No. 4,085,335 describes an optical parametric device wherein the coupling means, or arrangement in the parametric system, includes a dichroic mirror located in the path of the optical resonator, which dichroic mirror is arranged at an inclination with respect to the path of radiation within the resonator. The mirror has a high reflectivity for the primary radiation and a high transmissivity for the secondary radiation.
U.S. Pat. No. 4,180,751 describes a mode-locked optical parametric oscillator apparatus wherein the OPO cavity length is substantially smaller than the pump laser cavity length and in which the oscillator mirrors are singly resonant at either the signal or idler (pulse) frequencies. This configuration is intended to generate non-resonated OPO pulses which replicate mode-locked pump pulses, and both sets of pulses couple to the resonated OPO pulse over a relatively wide tuning range without adversely affecting OPO operation.
Thus, it is seen that an OPO may be used to generate secondary radiation having a wavelength significantly offset from the primary radiation. In many instances, the weaker component (f.sub.i) of the secondary radiation is shunted, in that it is not of interest and is not independently tunable with respect to the stronger (f.sub.s ) component.
In certain applications, it would be desirable to generate an optical beam (secondary parametric radiation) having two independently tunable and efficient wavelength components. For instance, two-photon spectroscopy is an example of where this would be desirable. Reference is made to commonly-owned, copending U.S. patent application No. (Atty. Docket No. JEN-101), entitled MID-INFRARED LIGHT HYDROCARBON DIAL LIDAR, filed on even date herewith, wherein the utility of generating a single beam with two discrete components in the mid-infrared range is desirable. The first component is from 3.16 to 3.18 microns and may be suitable for atmospheric methane detection. The second, is between 3.38 and 3.51 microns, where ethane, propane and butane can be detected.
Clearly, two independently tunable primary (signal) frequencies could be obtained with OPOs by splitting the incident radiation from a single laser source into two paths, each interacting with a separate OPO, and recombining the outputs of the OPOs. However, the optics required to form two OPO resonators, as well as the beam splitting and recombining optics, would be prohibitively expensive. Furthermore, the energy of the incident beam would be split and shared by the two discrete OPOs, and higher laser power would be required to achieve the same effect in each of the secondary radiation beams. This too would add to the cost of such a system.